Unsaturated carboxylic acids, such as acrylic acid and methacrylic acid, are industrially important as starting materials for various synthetic resins, coating materials and plasticizers. Two step vapor phase reaction processes from alkenes have historically been practiced for the production of unsaturated carboxylic acids, including acrylic acid and methacrylic acid and these processes are still widely used today. These two step reaction processes typically include a first reaction step wherein an alkene, such as propylene, is converted to an intermediate hydrocarbon product, such as acrolein, and a second reaction step wherein the intermediate hydrocarbon product, such as acrolein, is converted to an unsaturated carboxylic acid, such as acrylic acid. However, in view of the price difference between propane and propene, attention has been drawn to the development of a method for producing acrylic acid and methacrylic acid by using a lower alkane, such as propane, as the starting material, and catalytically reacting the lower alkane in a gaseous phase, in the presence of a suitable mixed metal oxide catalyst. Thus, recently, processes involving single-step vapor phase catalytic oxidation of alkanes, alkenes, and mixtures thereof, to produce unsaturated carboxylic acids have been researched. More particularly, such methods involve subjecting an alkane, an alkene, or a mixture thereof, to a vapor phase catalytic oxidation reaction in the presence of a suitable mixed metal oxide catalyst, to produce the corresponding unsaturated carboxylic acid.
Unsaturated nitriles, such as acrylonitrile and methacrylonitrile, have been industrially produced as important intermediates for the preparation of fibers, synthetic resins, synthetic rubbers, and the like. The most popular method for producing such nitrites is to subject an olefin, such as propene, to a catalytic reaction with ammonia, in the presence of a suitable catalyst in a gaseous phase at a high temperature. However, in view of the price difference between propane and propene or between isobutane and isobutene, attention has been drawn to the development of a method for producing acrylonitrile or methacrylonitrile by using a lower alkane, such as propane or isobutane, as the starting material, and catalytically reacting the lower alkane with ammonia in a gaseous phase, in the presence of a suitable mixed metal oxide catalyst.
Although in most cases, the exact catalyst formulations of the most suitable catalysts for use in the commercial processes mentioned above are proprietary to the catalyst supplier, the technology is well established and such catalysts are commercially available from various sources.
While a portion of the efforts to develop efficient and economical single-step vapor phase catalytic oxidation processes for the production of unsaturated carboxylic acids and unsaturated nitrites from their corresponding alkanes and alkenes has focused on identifying and preparing suitable catalysts, the possibility also exists for improvements to the overall reaction processes which would further increase yield and conversion rates by overcoming various process limitations.
More particularly, one such process limitation concerns the potential for gaseous streams containing hydrocarbons, such as alkanes and alkenes, and oxygen to autoignite, or spontaneously explode, when the concentrations of hydrocarbons and oxygen in the same gaseous stream are too high. In order to avoid such mishaps, the amounts of alkanes, alkenes and oxygen in gaseous feed streams of such oxidation processes are typically maintained below certain levels, which depend upon the particular constituents of the feed streams and the temperatures at which they exist in the processes. This generally means that in order to maximize the amount of hydrocarbons in a given stream, the stochiometric amount of oxygen required to convert all of the hydrocarbon in the stream cannot be added to the feed stream at the beginning of the process due to flammability concerns and, therefore, a significant amount of unreacted hydrocarbon is likely to be present in the product stream. One result of these circumstances is that the processes are inefficient, i.e., wasteful of hydrocarbon feed and achieving less than optimal product yields.
To avoid the aforesaid problem of creating flammable gaseous feed streams in vapor phase oxidation reactions, it is known to employ a multi-stage reaction system, i.e., having at least two oxidation reactors, or stages, in series with one another, and an oxygen feed arrangement which is explained and referred to hereinafter as “staged oxygen arrangement”. Such “staged oxygen arrangements” are discussed in connection with vapor phase oxidation reactions in, for example, U.S. Pat. No. 6,166,263, U.S. Pat. No. 4,899,003, and International Patent Application No. WO 01/98247. In addition, EP 1 081 124 A2 discloses a modified type of staged oxygen feed wherein the oxygen-containing feed stream is divided into a plurality of oxygen-containing streams that are successively introduced into a single reaction zone to facilitate the continuous conversion by catalyzed oxidation of the hydrocarbon feed.
In a reaction system having two oxidation reactors arranged in series with one another, the feed stream to the first oxidation reactor contains hydrocarbons and a less than stoichiometric amount of oxygen (such that the mixture is non-flammable), and the resulting effluent stream contains unreacted hydrocarbons, as well as by-products, along with the desired oxidation product(s). The effluent stream of the first oxidation reactor is fed to the second oxidation reactor, along with additional fresh oxygen, either pure oxygen or as part of an oxygen-containing stream (such as air) (such that the mixture is non-flammable), whereby at least some of the unreacted hydrocarbons in the stream are oxidized in the second oxidation reactor. As a result, the effluent of the second oxidation reactor contains more oxidation product and less unreacted hydrocarbon than the effluent stream of the first oxidation reactor. In addition, the staged oxygen arrangement also allows the concentration of hydrocarbons in the initial feed stream to the first oxidation reactor to be higher than that for a process arrangement without staged oxygen.
A problem has been encountered, however, relating to certain multi-stage vapor phase reaction systems that utilize a staged oxygen arrangement to enable higher hydrocarbon feed concentrations. More particularly, while the concentration of oxidation product in the effluent streams increases with each successive stage in such a multi-stage process, the overall process oxidation product yield is actually less than the yield for a comparable single stage reaction system. This problem must be addressed in order for such processes to recapture the benefits of the staged oxygen arrangement.
It is known in the art to use condensers to remove or separate certain condensable constituents from gaseous process streams, including intermediate and final stage effluent streams. For example, U.S. Pat. No. 6,166,263 discloses the use of a condenser in a single stage reaction to separate a vapor phase reaction effluent stream into a liquid product stream and a recycle gas stream which is recycled back to the reactor. WO 01/98247 discloses the use of a condenser in a two-stage vapor phase oxidation process to separate the effluent stream of the second reactor (i.e., the final stage effluent stream) into an off-gas stream, which is recycled back to the first reactor, and an aqueous carboxylic acid product stream.
U.S. Pat. No. 4,899,003 discloses the use of a condenser in a two-stage vapor phase oxidation process to treat an intermediate process stream. More particularly, the condenser removes a particular by-product (i.e., water) from the effluent of the first reactor before the effluent is fed, with additional oxygen, to the second reactor. The fact that a small amount of acrylic acid, another by-product, is also removed from the intermediate stream by the condenser is inconsequential to the goal of the disclosed process. The stated purpose of removing the water in U.S. Pat. No. 4,899,003 is to minimize the water present in the second reactor, whereby the vapor phase oxidation reaction in the second reactor is driven toward the desired product, ethylene, and away from production of by-product acetic acid.
WO 02/00587 discloses a vapor phase reaction process for converting a feed stream containing propylene and propane to hydroformylation products, such as butyraldehyde. This process involves partial condensation to separate the hydroformylation reaction effluent stream into a stream containing primarily the hydroformylation product and another gaseous stream containing primarily unreacted propylene and propane. The gaseous stream containing unreacted propylene and propane is fed to another vapor phase reaction process for producing different products, i.e., acrolein and acrylic acid. Thus, WO 02/00587 teaches subjecting the effluent stream of a first reaction process to partial condensation to recover the desired product and also to create another separate stream for feeding to an entirely separate, subsequent reaction process which produces different products.
The present invention provides a process which addresses the problem of a decreased total yield of oxidation product in multi-stage vapor phase oxidation reactions which employ staged oxygen arrangements for conversion of lower alkanes and alkenes, and mixtures thereof, to unsaturated carboxylic acids and/or unsaturated nitrites. More particularly, it has been discovered that in such processes, the removal of at least a portion of the oxidation product from each intermediate effluent stream, for example, by inter-stage partial condensation, prior to adding more oxygen and feeding the effluent stream to the next stage, unexpectedly results in overall cumulative oxidation product yields greater than either the original single-stage system or the system including only staged oxygen arrangements.